Effects of local tree diversity on herbivore communities diminish with increasing forest fragmentation on the landscape scale.

Forest fragmentation and plant diversity have been shown to play a crucial role for herbivorous insects (herbivores, hereafter). In turn, herbivory-induced leaf area loss is known to have direct implications for plant growth and reproduction as well as long-term consequences for ecosystem functioning and forest regeneration. So far, previous studies determined diverging responses of herbivores to forest fragmentation and plant diversity. Those inconsistent results may be owed to complex interactive effects of both co-occurring environmental factors albeit they act on different spatial scales. In this study, we investigated whether forest fragmentation on the landscape scale and tree diversity on the local habitat scale show interactive effects on the herbivore community and leaf area loss in subtropical forests in South Africa. We applied standardized beating samples and a community-based approach to estimate changes in herbivore community composition, herbivore abundance, and the effective number of herbivore species on the tree species-level. We further monitored leaf area loss to link changes in the herbivore community to the associated process of herbivory. Forest fragmentation and tree diversity interactively affected the herbivore community composition, mainly by a species turnover within the family of Curculionidae. Furthermore, herbivore abundance increased and the number of herbivore species decreased with increasing tree diversity in slightly fragmented forests whereas the effects diminished with increasing forest fragmentation. Surprisingly, leaf area loss was neither affected by forest fragmentation or tree diversity, nor by changes in the herbivore community. Our study highlights the need to consider interactive effects of environmental changes across spatial scales in order to draw reliable conclusions for community and interaction patterns. Moreover, forest fragmentation seems to alter the effect of tree diversity on the herbivore community, and thus, has the potential to jeopardize ecosystem functioning and forest regeneration

Data from: Natural habitat loss and exotic plants reduce the functional diversity of flower visitors in a heterogeneous subtropical landscape

1. Functional diversity (FD) of pollinators can increase plant reproductive output and the stability of plant-pollinator communities. Yet, in times of worldwide pollinator declines, effects of global change on pollinator FD remain poorly understood. Loss of natural habitat and exotic plant invasions are two major drivers of global change that particularly threaten pollinator diversity. 2. In a subtropical South African landscape, we investigated changes in the FD of flower visitor assemblages on native and exotic plants along gradients of natural habitat loss and relative abundance of exotic plants. We used a dataset of 1434 flower visitor individuals sampled on 131 focal plants and calculated the FD in three flower visitor traits that are strongly related to plant-flower visitor interactions and pollination processes: proboscis length, proboscis diameter and body length. 3. Multivariate FD of flower visitors decreased with both increasing natural habitat loss and relative exotic abundance. Importantly, changes in FD went beyond those in flower visitor richness. Furthermore, flower visitor richness was not related to either natural habitat loss or relative exotic abundance. Loss in multivariate FD seemed to be mediated by complementary reductions of FD in proboscis length with natural habitat loss and of FD in body length with both global change drivers. Correspondingly, we recorded lower abundances of long-tongued flower visitors with natural habitat loss and reduced variance in body size with both drivers. In contrast, FD in proboscis diameter was unaffected by either driver. All effects of the two global change drivers were non-interactive. 4. Our results show that both natural habitat loss and exotic plants negatively affect flower visitor FD, which may imperil pollination of specialised plant species in degraded habitats. In contrast, flower visitor richness may not cover all facets of flower visitor FD that are relevant to pollination processes, and here future studies are needed. Distinct responses of visitor traits to the two drivers suggest limited options to infer relations of one trait to another. Finally, additive effects of natural habitat loss and exotic plant invasions highlight the need to consider multiple drivers of global change when investigating ecosystem processes at a community scale

Functional traits of flower visitors and plants and plant-flower visitor interactions across 17 study sites in a South African landscape

Data on functional traits of flower visitors and plants and on plant-flower visitor interactions across 17 study sites in a South African landscape. Study sites differed in their local degree of natural habitat loss and relative floral abundance of exotic plant species. Functional traits of flower visitors were used to calculate changes in the functional diversity of flower visitor communities with increasing amount of habitat loss and relative abundance of exotic plants. See the read me file for more detailed information

Effects of forest fragmentation and tree diversity on herbivore abundance and number of herbivore species.

<p>Light to dark grey shaded areas depict low to high values for (left panel) herbivore abundance and (right panel) number of herbivore species per tree species per study site based on model fit; small to large radii of circles depict low to high values of original data for herbivore abundance and number of herbivore species per tree species per study site.</p

Path model for relationships between forest fragmentation, tree diversity, herbivore community, and leaf area loss.

<p>Causal relationships between forest fragmentation, tree diversity, herbivore abundance, number of herbivore species, and leaf area loss. Values next to arrows give effect estimates; black estimates and solid arrows show significant effects, and stars demark the significance level (pMCMC: 0.050< * >0.010< ** >0.001< *** >0.000).</p

Effects of forest fragmentation and tree diversity on the community composition of herbivores.

<p>Ordination plot of herbivore species per tree species per study site along the spatial component (SC), the gradients of forest fragmentation (FF) and tree diversity (TD), and their interactive effect (IE). Black points display species scores (n = 87) and stars demark the significance level (pMCMC: 0.050< * >0.010< ** >0.001< *** >0.000). We used a Constrained Analysis of Principal Coordinates (CAP) for visualization only as Software R does not provide a function to plot results of the perMANOVA.</p

Habitat Characteristics of Forest Fragments Determine Specialisation of Plant-Frugivore Networks in a Mosaic Forest Landscape

<div><p>Plant-frugivore networks play a key role in the regeneration of sub-tropical forest ecosystems. However, information about the impact of habitat characteristics on plant-frugivore networks in fragmented forests is scarce. We investigated the importance of fruit abundance, fruiting plant species richness and canopy cover within habitat fragments for the structure and robustness of plant-frugivore networks in a mosaic forest landscape of South Africa. In total, 53 avian species were involved in fruit removal of 31 fleshy-fruiting plant species. Species specialisation was always higher for plants than for frugivores. Both species and network-level specialisation increased with increasing fruit abundance and decreased with increasing fruiting plant species richness and canopy cover within fragments. Interaction diversity was unaffected by fruit abundance and canopy cover, but increased slightly with increasing fruiting plant species richness. These findings suggest that especially the availability of resources is an important determinant of the structure of plant-frugivore networks in a fragmented forest landscape.</p> </div

Mean ± SE specialisation of frugivores and plants within plant-fugivore networks. (n = 9).

<p>Mean ± SE specialisation of frugivores and plants within plant-fugivore networks. (n = 9).</p

Effects of habitat characteristics and trophic level on the structure of plant-frugivore networks.

<p>Species specialisation (<i>d'</i>), network specialisation (<i>H₂'</i>), interaction diversity and robustness of plant-frugivore networks (n = 9) in relation to fruit abundance, fruiting plant species richness, canopy cover (%) and trophic level (plant <i>vs.</i> frugivore). To investigate effects on species specialization, trophic level was nested within plot in a linear mixed effect model; effects on all other dependent variables were analysed using linear models. Given are <i>R²</i>, <i>t</i> and <i>P</i> values, if applicable. Note: all significant or marginally significant <i>P values</i> are highlighted in bold.</p